The present invention relates to an apparatus for dehydrating wood veneer or removing water contained in veneer by mechanical squeezing and also to a method therefor.
Wood veneer has been used widely for production of various veneer laminated products such as plywood, laminated veneer lumber (LVL), etc. Multiple layers of veneer are assembled together with an adhesive into the form of a panel, a board or lumber. Prior to the assembling, sheets of green or undried veneer are dried to the desired level of moisture content. As a preparation for such drying, dehydration of veneer or removing part of the water contained in the veneer by mechanical compression or squeezing of the veneer across the thickness thereof has been practiced in some veneer or plywood mills.
A typical veneer dehydrating apparatus is disclosed by U.S. Pat. No. 6,505,658. The veneer dehydrating apparatus has a pair of rotatable dehydrating rollers disposed one above the other with the axes thereof extending parallel to each other and at least one of the rollers is positively driven. The paired upper and lower dehydrating rollers are spaced radially so that a nip is formed between the peripheries thereof. The nip or the smallest spaced distance between the peripheral surfaces of the paired upper and lower rollers is smaller than the thickness of the veneer sheet to be dehydrated so that the veneer is squeezed by the rollers and part of the water contained in the veneer is removed.
In some veneer dehydrating apparatuses, the dehydrating rollers disposed one above the other are spaced at a spaced distance between the peripheral surfaces thereof that accounts for about 50% of the thickness of veneer sheet for dehydration. In dehydrating veneer by compression or squeezing, however, the presence of a relatively hard knot in wood veneer poses a problem.
Generally, the elastic range in the grain direction of a veneer sheet subjected to compression is much smaller than the elastic ranges in the tangential direction, i.e. the direction across the grain direction, and in the radial direction, i.e. the thickness direction. A knot in a veneer sheet, which is originally the base of a side branch of a tree, has its grain oriented across the grain of the veneer sheet. Specifically, the grain of a knot in a veneer sheet is oriented in the thickness direction of the veneer sheet. Therefore, when a veneer sheet having therein any knot is passed between the dehydrating rollers and compressed across the thickness by such rollers, the strain of the knot easily exceeds its elastic-plastic range, with the result that the knot is broken into small pieces and a void is formed through the veneer sheet where the knot was present. A veneer having such a void or open hole is graded as low quality veneer and the veneer laminated product having therein such veneer is regarded as low quality product, accordingly. Though the spaced distance between the dehydrating rollers may be enlarged with an attempt to protect the knot against breakage, the reduced compression due to the increased spaced distance may nullify the dehydrating effect of the apparatus.
Therefore, the present invention which has been made in light of the above problem, is directed to proving a veneer dehydrating method and apparatus that offer effective dehydration for veneer having therein a knot.